no permission when using AT for backup

Hi
I'm using the AT command to run a batch file that backs the local c drive and the c$ shares on three remote servers onto a tape drive on the local server. The problem is when AT runs the batch file it gives permission denied error messages onall of the mapped c$ drives. If the batch file is run manually no such permission problems occur and the backup runs as nornal. Does anyone know why this is happening and how I can get around this using AT. Thanks in advance